Abstract:

A color mapping system comprises a detail detector (1) to generate a
control signal (CS) which indicates local detail in an input image signal
(IS). The system further comprises a color mapper (2) which maps a first
image signal (FIS) into a mapped image signal (MIS) under control of the
control signal (CS) for locally changing an intensity and/or a saturation
of the first image signal (FIS) as a function of the local detail. The
first image signal (FIS) is the input image signal (IS) or a low-pass
filtered input image signal (LIS).

Claims:

1. A color mapping system comprising:a detector (1) arranged to analyze a
local image structure in an image (IS) and to output a image structure
measure usable for generating a control signal (CS) indicating a type of
local image structure in the image (IS),a color mapper (2) for mapping a
first image signal (FIS) into a mapped image signal (MIS) by means of a
color transformation under control of the control signal (CS), such as
for locally changing an intensity and/or a saturation of the first image
signal (FIS) as a function of the local image structure.

2. A color mapping system as in claim 1 comprising:a detail detector (1)
for generating a control signal (CS) indicating local detail in an input
image, the input image being defined by an input image signal (IS),a
color mapper (2) for mapping a first image signal (FIS) into a mapped
image signal (MIS) by means of a color transformation under control of
the control signal (CS), such as for locally changing an intensity and/or
a saturation of the first image signal (FIS) as a function of the local
detail, wherein the first image signal (FIS) is the input image signal
(IS) or a filtered input image signal (LIS).

3. A color mapping system as claimed in claim 1, wherein the color mapper
(2) is constructed for generating an intensity change of unsaturated
colors.

4. A color mapping system as claimed in claim 3, wherein the color mapper
(2) is constructed for generating the intensity change of the unsaturated
colors to locally decrease the intensity as a function of the increase of
the local detail, or to locally increase the intensity as a function of
the increase of the local detail.

5. A color mapping system as claimed in claim 2, wherein the color mapper
(2) is constructed for locally decreasing a saturation of saturated
colors as a function of the increase of the local detail.

6. A color mapping system as claimed in claim 2, wherein the detail
detector (1) is constructed for generating the control signal (CS)
indicating the local detail of a chrominance component of the input image
signal (IS).

7. A color mapping system as claimed in claim 6, wherein the detail
detector (1) comprises:a high pass filter (10) for supplying a high-pass
filtered image signal (HFI) being a high-pass filtered input image signal
(IS),a chrominance detail detector (11) for receiving the high-pass
filtered image signal (HFI) to determine a local difference (LDC) of
chrominance values within an area of the input image signal (IS), the
area including a presently to be color mapped pixel of the input image
signal (IS), anda control signal generator (12) for receiving the local
difference (LDC) to generate the control signal (CS) indicating the local
amount of chrominance detail.FIGS. 1, 3 and 4

8. A color mapping system as claimed in claim 1, wherein the color mapped
image signal (MIS) has a second gamut (GA2) being larger than a first
gamut (GA1) of the first image signal (FIS).

9. A color mapping system as claimed in claim 8, wherein the first gamut
(GA1) is defined by three primaries (R, G, B) and the second gamut (GA2)
is defined by the three primaries (R, G, B) and a white primary (W).

10. A color mapping system as claimed in claim 2, wherein the color
mapping system comprises a low-pass filter (4) for receiving the input
image signal (IS) to supply the first image signal (FIS) being low-passed
filtered.

11. A color mapping system as claimed in claim 10, wherein the low-pass
filter (4) is an adaptive low-pass filter (4) being coupled to the detail
detector (1) for increasing its amount of low-pass filtering as a
function of an increasing detail.

12. A color mapping system as claimed in claim 11, wherein the adaptive
low-pass filter (4) comprises:a low-pass filter (101) for receiving the
input image signal (IS) to supply a third image signal (TIS), anda
combiner (41) for supplying the low-pass filtered input image signal
(LIS) being a weighted combination of the input image signal (IS) and the
third image signal (TIS).

13. A color mapping system as claimed in claim 1, wherein the first image
signal (FIS) is the input image signal (IS), and wherein the conversion
system further comprises:a low-pass filter (101) for receiving the input
image signal (IS) to supply a third image signal (TIS),a combiner (6) for
supplying an output image signal (SIS) being a weighted combination of
the third image signal (IS) and the mapped image signal (MIS).

14. A conversion system for converting an M-primary image signal (R, G, B)
into an N-primary image signal (R, G, B, W) wherein N is greater than M,
the conversion system comprises:the color mapping system as claimed in
claim 6 wherein both the first image signal (FIS) and the mapped image
signal (MIS) are M-primary image signals, anda multi-primary converter
(3) for converting the mapped image signal (MIS) into the N-primary image
signal (NIS).

15. A conversion system for converting an M-primary image signal (R, G, B)
into an N-primary image signal (R, G, B, W) wherein N is greater than M,
the conversion system comprises:the color mapping system as claimed in
claim 11 wherein both the first image signal (FIS) and the mapped image
signal (MIS) are M-primary image signals, anda multi-primary converter
(3) for converting the output image signal (SIS) into the N-primary image
signal (NIS).

17. A color mapping method comprising:generating a control signal (CS)
indicating local image structure in an input image signal (IS), andcolor
mapping (2) a first image signal (FIS) into a mapped image signal (MIS)
under control of the control signal (CS) for locally changing an
intensity and/or a saturation of the first image signal (FIS) as a
function of the local image structure.

18. A computer program product comprising computer code for performing the
steps of:generating a control signal (CS) indicating a local image
structure in an input image signal (IS),color mapping (2) a first image
signal (FIS) into a mapped image signal (MIS) under control of the
control signal (CS) for locally changing an intensity and/or saturation
of the first image signal (FIS) as a function of the local image
structure.

Description:

FIELD OF THE INVENTION

[0001]The invention relates to a color mapping system, a conversion system
for converting an M-primary image signal into an N-primary image signal,
a display apparatus, a color mapping method, and a computer program
product.

BACKGROUND OF THE INVENTION

[0002]Gamut mapping is known from systems which have an input image signal
defined in an input gamut which is different than an output gamut of a
display device on which the image has to be displayed. For example for an
RGBW (Red, Green, Blue, White) display which has pixels each comprising a
red, green, blue and white sub-pixel, a gamut mapping maps the standard
RGB (Red, Green, Blue) input signal into a mapped image signal which can
be displayed on the sub-pixels of the RGBW display. The sub-pixels, emit
light with corresponding colors referred to as the display primaries.
Usually, this mapping only involves the process of determining how the
colors in the input color space defined by the input image signal RGB
have to be mapped in the input color space to colors which fit the output
gamut defined by the RGBW primaries. A successive multi-primary
conversion converts the mapped colors to drive signals for the RGBW
sub-pixels. The operation of the prior art gamut mapping and
multi-primary conversion will be discussed in more detail with respect to
FIGS. 2A to 2C. It is a drawback of the known color mapping or gamut
mapping systems that artifacts occur for particular input image
structures.

SUMMARY OF THE INVENTION

[0003]It is an object of the invention to improve the picture quality of
the color mapped image signal.

[0004]A first aspect of the invention provides a color mapping system as
claimed in claim 1. A second aspect of the invention provides a
conversion system as claimed in claim 13. A third aspect of the invention
provides a display apparatus as claimed in claim 15. A fourth aspect of
the invention provides a color mapping method as claimed in claim 16.

[0005]A fifth aspect of the invention provides a computer program product
as claimed in claim 17. Advantageous embodiments are defined in the
dependent claims.

[0006]A color mapping system in accordance with the first aspect of the
invention comprises a detail detector which generates a control signal
indicating a local detail in an input image signal. With detail should be
understood the local image structure, i.e. not necessarily the presence
of a high frequency local pattern, but also the absence of it, i.e. e.g.
a uniform region, possibly apart from some noise (in this text we will
usually mean with detail small grain or high frequency detail). The term
color mapping is used to indicate any mapping of colors of an input image
into colors of an output image, independent on whether the input and
output gamuts are different or not. Gamut mapping is considered to be a
special case wherein the color mapping occurs for different gamuts. Due
to the color mapping, at least one color of the input signal is mapped on
a different color at the output of the color mapper. With color is meant
luminance, saturation, and/or hue.

[0007]The input image signal has images composed of pixels. The color and
intensity of each one of the pixels is defined by input signal samples
which comprise components which directly (RGB) or indirectly (YUV) define
the intensity of each one of the primaries used for representing the
input image signal. For full color images, at least three differently
colored primaries are required. These primaries define the gamut of the
input signal. An image may be a photo, a picture of a film, or a computer
generated image which may be a composition of text and photo and/or film.

[0008]The detail detector checks for each pixel of the input image the
detail present in a local area including the pixel. For example, the
difference between the sample of a previous pixel and the sample of the
present pixel which has to be color mapped is determined. The higher this
difference is the more high frequent detail is present. This difference
may be determined from the differences of all or particular components of
the samples. For example if the local chrominance detail should be
determined, the differences of the chrominance components of input sample
adjacent to the presently to be processed input sample may be determined.
Alternatively, more than one pixel on the same line as the presently to
be processed pixel may be used to determine the local detail. The local
area may also include pixels of preceding and/or succeeding lines. It has
to be noted that the local detail is interpreted to be any local
structure. The amount of local detail increases if more detail or
structure is present in a predefined area, and/or if more high frequent
detail is present in the predefined area.

[0009]The color mapper (or color map unit) maps an image signal into a
mapped image signal under control of the control signal. The control
signal locally changes the intensity and/or the saturation of the image
signal as a function of the local detail detected. Consequently, if an
artifact is caused which depends on the intensity or the saturation of
the present pixel and which is dependent on the local detail at the
present pixel, the change of the intensity or the saturation dependent on
the local detail decreases the visibility of the artifact.

[0010]In an embodiment, the control signal steers the local intensity
change of unsaturated colors by the color mapper. If the color mapper
maps from a particular color gamut to a larger color gamut, the control
signal causes the color mapper to locally decrease an intensity boosting
if much local detail is present. With a larger color gamut is meant a
color gamut which provides a larger luminance range which usually occurs
if more primaries are used. Or said differently, the intensity boosting
is decreased as a function of an increase of the local detail. If the
mapper maps from a particular color gamut to a smaller color gamut,
usually, the control signal causes the color mapper to locally decrease
an intensity decrease if much local detail is present. Or said
differently, the intensity decrease is decreased as a function of an
increase of the local detail. The detail controlled color mapping can
also be implemented in systems wherein the input gamut and the output
gamut are identical. The image signal received by the color mapper may be
the same input image signal as received by the detail detector, but
alternatively may be a filtered input image signal. For example, a
low-pass filter, which may be adaptive or is an anti-aliasing filter. The
filter may be linear or non-linear and is constructed to prevent
artifacts occurring is the successive sub-pixel mapping.

[0011]Consequently, if much detail is present in the signal to be mapped,
the prior art mapping applies the same mapping, for example an intensity
boost, as if no detail is present. For particular input image content,
such as for example a thin saturated red line in a green background
whereby unsaturated red lines are flanking the red line, artifacts occur
if the standard high amount of intensity boost is applied. The
unsaturated red lines are intensity boosted and thus are brighter in the
mapped signal than in the input signal. The saturated red line cannot be
boosted and thus keeps its original color and intensity. The effect of
the color mapping is that the thin red line becomes much broader.
Consequently, the color mapping results in a loss of detail in the
displayed image.

[0012]The color mapping system in accordance with this embodiment of the
present invention detects the high frequent information in the area
comprising the thin red line and locally decreases its intensity boost.
Thus, the unsaturated red color of the flanking lines changes less
towards the color of the saturated red line than in the prior art or even
not at all. Consequently, the detail in the input image is preserved in
the mapped image. On the other hand, for areas where no detail is
present, the prior art intensity boost can be applied without creating
artifacts. To conclude: the detail adaptive color mapping in accordance
with the present invention has the advantage that the same intensity
boosting is obtained as in prior art color mappings in areas with a low
amount of detail, while the artifacts in areas with a high amount of
detail are decreased.

[0013]In an embodiment, the color mapper locally decreases the saturation
of saturated colors as a function of the increase of the local detail up
to a predefined amount. By lowering the saturation, artifacts caused by a
subsequent sub-pixel rendering are decreased. This is illustrated, by way
of example, for an RGBW display. The display of a saturated image area on
a RGBW display is only possible by driving the RGB sub-pixels. The W
sub-pixel cannot be used because the saturated image area would become
de-saturated. For example for a fully saturated yellow area, only the R
and G sub-pixels are driven to emit light, the B and W sub-pixels do not
emit light. For large uniform areas this does not cause any problem.
However, for example, a drastic artifact occurs if a thin black line is
present in a saturated yellow background. Either, a black pixel of the
black line is mapped on an RGB sub-pixel group or on a W sub-pixel. If
the pixel falls on a RGB sub-pixel group, the line appears broader
because the adjacent W sub-pixel also does not emit light. If the pixel
falls on a W sub-pixel, the black pixel gets lost because all the W
sub-pixels did already not emit light, while the adjacent RGB sub-pixel
group is used to generate the yellow light.

[0014]This prior art problem can be alleviated by de-saturating the input
signal under control of the detail detected. If no detail is detected, no
de-saturation is required and the saturated color of the uniform area is
kept saturated. If detail is detected, the saturated color is
de-saturated and consequently, the W sub-pixels are able to display
information thereby decreasing the artifacts caused by the switched-off W
sub-pixels. The thin black line becomes more visible, be it on a less
saturated background.

[0015]The amount of de-saturation may be dependent on the detail. For
example, the amount of de-saturation may increase with increasing detail
until a predetermined level of detail. This predetermined level of detail
may be the maximum chrominance detail which the display is able to
display. If the predetermined level of detail is not the maximum
chrominance detail and the detail rises above the predetermined level,
the de-saturation decreases with increasing detail.

[0016]The de-saturation may be obtained by mixing the luminance intensity
of the input RGB pixel with the input sub-pixel intensities R, G, B. The
mixing may be a linear addition using weight factors. The weight factors
may be controlled by the local detail detected. Alternatively, the
average value of the R, G, B sub-pixel intensities is mixed with the
individual R, G, B, sub-pixel values. Alternatively, luminance detail
(high pass filtered luminance of the input signal) may be added instead
of the luminance itself.

[0017]Of course, this approach works also for RGBX displays wherein X is
an additional primary color, or for any multi-primary display.

[0018]In an embodiment the detail detector detects the local detail in the
chrominance of the input image signal. For example, the detail in the UV
components may be determined. The UV signals may be directly available if
the input signal is a YUV signal or may be calculated if the input signal
is a RGB signal. This is especially relevant if the artifacts depend on
the chrominance of the input image signal samples.

[0019]In an embodiment, the detail detector comprises a high pass filter
to supply a high-pass filtered image signal which is a high-pass filtered
version of the input image signal. A chrominance detail detector receives
the high-pass filtered image signal to determine a local difference of
chrominance values within an area of the input image signal. The area
includes the pixel of the input image signal which has be color mapped. A
control signal generator receives the local difference to generate the
control signal indicating the local amount of chrominance detail.

[0020]In an embodiment, the color mapped image signal has a gamut which is
larger (brighter) than a gamut of the first image signal. This is true,
for example, for a RGB to RGBW mapping. A color mapping which boost the
intensity of unsaturated colors is advantageously implemented in systems
wherein the gamut is increased. Such a color mapper is particularly
relevant in systems wherein the display gamut is larger than the gamut of
the input image signal. For example, usually, the input image signal is
defined in the EBU RGB (Red, Green, Blue) gamut while the display pixels
comprise, besides the conventional RGB sub-pixels, an additional
sub-pixel which for example emits white or yellow light. The addition of
the white primary enables to maximally increase the intensity of
unsaturated colors.

[0021]In an embodiment, the color mapping system comprises a low-pass
filter which receives the input image signal and which supplies the
low-passed input image signal to the mapper. Such a low-pass filtering is
especially advantageous if the display resolution is lower for
chrominance than for luminance. This is for example true for
configurations with RGBW sub-pixels, such as for example a pentile pixel
structure. It has to be noted that the use of a low-pass filter causes
smearing of a thin saturated line. In fact, the thin saturated line will
be flanked by unsaturated lines. If the prior art color mapping is
applied on these smeared lines, as is discussed hereinbefore the detail
gets lost. If the color mapping in accordance with the present invention
is combined with the low-pass filter, the intensity boosting of the
unsaturated lines is decreased decreasing the resolution loss in the
color mapped image.

[0022]In an embodiment wherein the mapper receives the low-pass filtered
input image signal, the low-pass filter is an adaptive low-pass filter
which increases its low-pass filtering as a function of an increasing
detail. Thus, the same detail detector as used for the mapping can be
used to control the adaptive low-pass filtering.

[0023]In an embodiment wherein the mapper receives the low-pass filtered
input image signal, the adaptive low-pass filter, which low-pass filters
the input image to obtain a low-pass filtered input image signal,
comprises a low-pass filter and a combiner. The low-pass filter low-pass
filters the input image signal to obtain a filtered image signal. The
combiner determines the low-pass filtered input image signal as a
weighted combination of the input image signal and the filtered image
signal. The weighting is controlled in function of the local detail
detected. The more weight is allocated to the low-pass filtered signal
the more detail is detected.

[0024]In an embodiment, the input image signal of the color mapper is
identical to the input image signal of the detail detector. The
conversion system comprises a low-pass filter which low-pass filters the
input image signal to obtain a low-pass filtered image signal. A combiner
determines the output image signal as a weighted combination of the
low-pass filtered image signal and the mapped image signal. The more
weight is allocated to the low-pass filtered signal the more detail is
detected. Thus, in local areas with a high amount of detail, the mapped
image signal does not or only minimally contribute to the output signal.
Consequently, the artifacts caused by the mapper will be minimally added
to the output signal.

[0025]In an embodiment, the conversion system converts an M-primary image
signal into an N-primary image signal, wherein N is greater than M. The
conversion system comprises the color mapping system and the
multi-primary converter. In the color mapping system both the image
signal received by the mapper, and the mapped image signal are M-primary
image signals. The multi-primary converter converts the M-primary mapped
image signal into the N-primary drive image signal. Such a system has the
advantage that the color mapping and the multi-primary conversion are
separated and thus can be optimized separately.

[0026]In an embodiment, the conversion system converts an M-primary image
signal into an N-primary image signal, wherein N is greater than M. The
conversion system comprises the color mapping system wherein both the
first image signal and the mapped image signal are M-primary image
signals, and a multi-primary converter for converting the output image
signal which is a combination of the low-pass filtered image signal and
the mapped image signal into the N-primary image signal.

[0027]These and other aspects of the invention are apparent from and will
be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]In the drawings:

[0029]FIG. 1 schematically shows a basic block diagram of a conversion
system which converts an M-primary image signal into an N-primary image
signal,

[0030]FIGS. 2A to 2C schematically show drawings illustrating the mapping
and the multi-primary conversion,

[0031]FIG. 3 schematically shows a block diagram of an embodiment of the
color mapping system wherein the adaptive low-pass filter and the
adaptive color mapper are arranged in series,

[0032]FIG. 4 schematically shows a block diagram of an embodiment of the
color mapping system wherein the adaptive low-pass filter and the
adaptive color mapper are arranged in parallel,

[0033]FIG. 5 schematically shows a block diagram of an embodiment of the
color mapping system further performing a detail controlled
de-saturation,

[0034]FIGS. 6A to 6C schematically show an embodiment of mixing factors in
the block diagram of FIG. 5,

[0035]FIG. 7 schematically shows a conversion from RGB input samples of
the input image into drive values of pentile structured sub-pixels of a
display, and

[0037]It should be noted that items which have the same reference numbers
in different Figures, have the same structural features and the same
functions, or are the same signals. Where the function and/or structure
of such an item has been explained, there is no necessity for repeated
explanation thereof in the detailed description.

DETAILED DESCRIPTION

[0038]FIG. 1 schematically shows a basic block diagram of a conversion
system which converts an M-primary image signal into an N-primary image
signal. A color mapper 2 maps its M-primary input image signal FIS into
an M-primary mapped image signal MIS. The multi-primary converter 3
converts the M-primary mapped image signal MIS into the N primary image
signal NIS. For example, the M-primary input image signal FIS comprises a
sequence of input samples which each comprise three components
representing three primary colors. The three primary colors usually are
red green and blue and are represented by a RGB signal, but may be
represented by another signal such as a YUV signal. The input gamut
comprises all possible colors (hue, saturation and intensity) which can
be represented by the input primary colors. The N primary image signal
NIS may be intended for driving N sub-pixels of a pixel of the display on
which the image should be displayed. In a RGBW display which has red,
green, blue and white sub-pixels, N=4. The output gamut comprises all
possible colors which can be represented by the display. In this example
wherein a RGB input signal is converted into RGBW display drive signals,
the input gamut is smaller than the output gamut. Consequently, the
mapper has to perform an intensity boost on unsaturated colors to be able
to fill the larger output gamut. The multi-primary converter converts the
colors in the mapped image, which are still represented with respect to
the input primaries RGB to the drive values RGBW for the display. Such a
mapper and multi-primary converter are well known.

[0039]In accordance with the present invention, the color mapping system,
or the conversion system, which further comprises the detail detector 1
which determines a local detail in the input image signal IS. Thus, in
accordance with the present invention, the color mapping system comprises
the color mapper 2 and the detail detector 1 but no multi-primary
converter 3, while the conversion system further comprises the
multi-primary converter 3. The local detail is the detail in a local area
of the input image signal IS including the input sample to be converted
or to be color mapped. In fact, it is meant that the detail is determined
based on input samples which correspond to pixels of the image which
occur in the local area. The color mapper 2 is now constructed to perform
the intensity boost of the unsaturated colors under control of the local
detail detected. The intensity boost is decreased the more detail is
detected. Thus, if the difference between closely spaced input samples is
large, the intensity boost of the unsaturated colors is small or even
zero. Consequently, the original differences are kept as much as
possible, thereby preventing a resolution decrease. On the other hand, in
areas wherein the differences between closely spaced input samples are
small, a large intensity boost can be applied resulting in a brighter
image without losing detail.

[0040]The input image signal IS of the detector 1 and the input image
signal FIS of the mapper 2 may be the same image signal, as will be
elucidated in more detail with respect to the embodiment of FIG. 4.
Alternatively, the input image signal FIS of the mapper 2 may be a
low-pass filtered version of the input image signal IS of the detector 1,
which will be elucidated in more detail with respect to the embodiment of
FIG. 3.

[0041]In the above example, wherein the output gamut is larger than the
input gamut, a mapper is discussed which maps unsaturated colors on other
colors by performing an intensity boost. However, in other systems
wherein the input gamut is wider than the output gamut, the mapper may
decrease the intensity of unsaturated colors, or may map colors outside
the output gamut into the output gamut in any other manner. Even if the
input and output gamut are identical, the color mapper may map particular
colors to other colors to improve the image in one way or another.

[0042]FIGS. 2A to 2C schematically show drawings illustrating the mapping
and the multi-primary conversion. In the example shown, for the ease of
explanation, the conversion system converts a two primary input signal
into a three primary display drive signal. Again, by way of example only,
the two primary input signal comprises a red R and a green G primary, and
the three primary drive signal comprises a red R, a green G and a yellow
Y primary.

[0043]FIG. 2A shows the color gamut GA1 comprising all colors of the input
samples of the input image signal FIS of the mapper 2. In a practical
implementation, the minimum and maximum values of the primary components
in the input image signal are limited due to physical constraints. For
example, the voltage swing is limited, or the number of bits used to
represent the primary components is limited. Therefore, both the
primaries R and G have normalized amplitudes in the range from zero to
one, including the borders of the range. A few samples P1 to P5 are
indicated in FIG. 2A to elucidate how these samples are mapped by the
mapper 2, and are converted by the multi-primary converter 3. The sample
P1 is black, the sample P2 is saturated green G with half intensity, the
sample P3 is near full saturated green G, and the sample P4 is yellow Y
with 3/4 intensity. The gamut GA1 comprises all the colors which can be
reproduced by varying the intensity of the R and G primaries between zero
and one.

[0044]FIG. 2B shows in the same R and G color space as shown in FIG. 2A a
gamut GA2 which can be realized if a yellow primary Y would be added
which is the sum of the R and G primaries. The mapper 2 implements an
algorithm which maps the input colors in FIG. 2A onto the possible colors
within the gamut GA2 of FIG. 2B. A very simple algorithm is to increase
for each color in FIG. 2A the values of the primaries R and G with a
factor two. Thus, in the example shown, an intensity boost with a factor
of two is obtained. Other factors for the intensity boost are possible.
The result would be a gamut spanned by primaries 2R and 2G as indicated
in FIG. 2B partly with dashed lines. However, as is clear from FIG. 2B,
the colors in the left top triangle (spanned by G, 2G, R) and in the
right bottom triangle (spanned by R, 2R, G) cannot be reproduced by the
sum of the primaries R, G and the primary Y. Therefore, usually, the
intensity boosting is not performed on the saturated colors on the G or R
axis but only on the unsaturated colors. Further a hard or soft clipping
is implemented for colors which occur after the intensity boosting within
the above mentioned triangles. For example, in FIG. 2B, the clipping
moves a color outside the gamut GA2 into this gamut.

[0045]The operation of the mapper 2 is now elucidated by discussing the
mapping of the samples P1 to P5 shown in FIG. 2A. The black sample P1 is
mapped to black P1'. The saturated green sample P2 is mapped to itself
and indicated by P2'. Of the unsaturated sample P4, the R and G values
are doubled such that the color P4' results within the gamut GA2.
However, if the R and G values of the unsaturated sample P3 are doubled,
the color P3' results which lies outside the gamut GA2. The color P3',
which cannot be reproduced in a system with the three primaries R, G and
Y, is, for example, hard clipped to the color P3'M on the border of the
gamut GA2. Thus, the color mapper 2 defines for all the colors of the
gamut GA1 how they are converted into colors within the gamut GA2. In
fact, the effect of the color mapping discussed is an intensity boosting
of non-saturated colors, while saturated colors (R and G) are kept
unchanged. It has to be noted that in prior art color mappers, usually a
user controllable factor is used instead of a fixed intensity boosting
factor of two. This factor may depend on the color of the primaries.

[0046]Although in the example shown, the gamuts GA1 and GA2 are different,
this is not essential. Alternatively, an image processing may involve a
color mapping between two identical gamuts or to a smaller gamut. If the
color mapping occurs to a smaller gamut, the intensity boosting may be an
intensity decrease. Thus, said more general, the color mapping changes
the intensity of unsaturated colors.

[0047]Now all colors are within the gamut GA2 which can be represented
with the three primaries R, G, Y, the actual multi-primary conversion
from the R, G color space to the R, G, Y color space has to be performed
such that the three drive signals of the three R, G, Y sub-pixels are
obtained. The multi-primary conversion is explained with respect to FIGS.
2B and 2C.

[0048]FIG. 2c shows in the R, G, Y color space two examples of many
possibilities of how the color P4' can be obtained by different
combinations of values of the three R, G, Y primaries. A first
possibility is to sum Y, bR and bG, and a second possibility is to sum
cY, aR and aG. Consequently, the task of the multi-primary converter 3 is
to select one out of the many possible different combinations. Usually,
the multi-primary converter performs this selection process under a
constraint, such as for example, to select, if possible, the sum for
which the luminance of the Y contribution is equal to the luminance of
the combined R and G contribution.

[0049]FIG. 3 schematically shows a block diagram of an embodiment of the
color mapping system wherein the adaptive low-pass filter and the
adaptive color mapper are arranged in series.

[0050]The detail detector 1 comprises a high-pass filter 10, a chrominance
detail detector 11 and a control signal generator 12. The high-pass
filter 10 comprises a low-pass filter 101 and an adder 102. The low-pass
filter 101 receives the input image signal IS to supply the low-pass
filtered image signal TIS. The adder 102 subtracts the low-pass filtered
image signal TIS from the input image signal IS to supply the high-pass
filtered image signal HFI. The chrominance detail detector 11 determines
the detail in the chrominance of the high-pass filtered image signal HFI.
The chrominance signal may be defined by U=R-G, and V=B-G. Now, the
chrominance detail detector 11 determines the delta(s) between U values
and V values, respectively, for sample values in the local area including
the present sample to be processed. The control signal generator 12
receives the delta values, which are also referred to as the local
difference LDC, to generate a control signal CS. The control signal CS
indicates the local chrominance detail. For example the control signal CS
comprises a factor k within the range from zero to one. The factor k
increases the more chrominance detail is detected. The low-pass filter
may have a one or two-dimensional kernel. The detector 11 may determine
instead of the chrominance detail the luminance detail or the total
detail in the input image signal IS.

[0051]The color mapper 2 in accordance with an embodiment of the present
invention comprises a prior art color mapper 20, a multiplier 21, a
multiplier 23 and an adder 22. For example, the prior art color mapper 20
performs the mapping as elucidated in FIGS. 2A and 2B. Usually, the color
mapper receives a user controllable factor which controls the amount of
intensity boost to be applied. In the embodiment shown in FIG. 3, this
factor is fixed, for example to its maximum value two. The image signal
LIS received by the color mapper 2 is mapped by the prior art color
mapper 20 to obtain an image signal I1. The multiplier 21 multiplies the
image signal I1 with the factor 1-k to obtain the image signal I2. The
multiplier 23 multiplies the image signal LIS, which is the input image
signal of the color mapper 20, with the factor k to obtain the image
signal I3. The adder 22 sums the image signal I2 and I3 to obtain the
mapped image signal MIS.

[0052]Thus, if much local detail is detected for the currently processed
input sample, the output signal of the color mapper 2 is multiplied by a
small value while the image signal LIS is multiplied by a value near to
one. Consequently, the mapped image signal MIS is almost identical the
input signal LIS of the mapper 2. If no or only a small amount (of high
frequent) local detail is detected, the value of the factor k is small
(near zero) and the value of the factor 1-k is near one. Consequently,
the mapped image signal MIS is almost identical to the prior art mapped
image signal I1.

[0053]In the embodiment shown in FIG. 3, the color mapper 2 receives an
adaptive low-pass filtered input image signal LIS. The adaptive low-pass
filter 4 comprises the low-pass filter 101, a multiplier 42, a multiplier
43 and an adder 41. The multiplier 42 multiplies the output image signal
TIS of the low-pass filter 101 with the factor k to obtain the image
signal I4. The multiplier 43 multiplies the input image signal IS with
the factor 1-k to obtain the image signal I5. The adder 41 sums the image
signals I4 and I5. Thus, if much local detail is detected, the image
signal LIS is equal to the low-pass filtered image signal TIS, and if no
local detail is present, the image signal LIS is equal to the input image
signal IS. Such an adaptive low-pass filter is especially advantageous if
the resolution of the display is higher for luminance than for
chrominance, which for example is true for a RGBW sub-pixel. For example,
a pentile structure is elucidated with respect to FIG. 5. For this kind
of displays, if is known that the luminance resolution of display is
sufficient to cater for the luminance resolution of the input signal, the
local detail detector 1 determines the local detail in the chrominance
only.

[0054]It has to be noted that the adaptive low-pass filter 4 as such is
known from the non pre-published European patent application 05110562.5
(or PCT application IB2006/054005).

[0055]FIG. 4 schematically shows a block diagram of an embodiment of the
color mapping system wherein the adaptive low-pass filter and the
adaptive color mapper are arranged in parallel. The detail detector 1
shown in FIG. 4 only differs from the detail detector 1 shown in FIG. 3
in that instead of the two factors k and k-1, now, optionally, three
factors k1, k2 and k3 are generated which have values dependent on the
local detail detected. In FIG. 4, both the detail detector 1 and the
color mapper 2 receive the input image signal IS as their input image
signal.

[0056]The color mapper 2 of this embodiment comprises a prior art color
mapper 20 and a multiplier 21. The multiplier 21 multiplies the color
mapped image signal I6 from the color mapper 20 with the factor k2 to
supply the mapped image signal MIS. Again, this factor k2 should take
care that the mapped image signal is suppressed more, i.e. the mapped
image signal MIS is closer to the input signal IS, the more local detail
is present in the input image signal IS.

[0057]The adaptive low-pass filter comprises the low-pass filter 101, the
multiplier 5, the optional multiplier 7, and the adder 6. The multiplier
5 multiplies the low-pass filtered image signal TIS with the factor k1 to
obtain the image signal I7. The factor k1 should increase with increasing
local detail. The multiplier 7 multiplies the input image signal IS with
the factor k3 to obtain the image signal I8. The factor k3 should
decrease with increasing local detail (and in general holds: k1+k2+k3=1).
The adder 6 adds the image signals I7 and I8 and MIS to supply the output
image signal SIS. In fact, the adaptive low-pass filter and the
controlled color mapper 2 of FIG. 3 are now arranged in parallel thereby
minimizing the number of adders and multipliers required.

[0058]First, the embodiment without the multiplier 7 is elucidated, the
factor k1 may be identical to the factor k in FIG. 3, and the factor k2
may be identical to the factor k-1 in FIG. 3. Thus, if much detail is
detected, the output image signal SIS is predominantly determined by the
low-pass filtered image signal TIS. If a low amount of detail is present,
the output image signal SIS is predominantly determined by the mapped
image signal MIS.

[0059]In the embodiment with the multiplier 7, it is possible to control
the amount of the low-pass filtered input image signal TIS, the mapped
input image signal MIS, and the input image signal IS itself as a
function of the local detail detected. For example, for a high amount of
local chrominance detail the factor k1 is 1 and the factors k2 and k3 are
0 such that the output image signal SIS is the low-pass filtered input
signal TIS. The low-pass filtering 101 may only be applied on the
chrominance components of the input signal IS. For a low amount of local
chrominance detail the factors k1 and k3 may be 0 and the factor k2 is 1.
The factor k3 may be non-zero for in-between amounts of chrominance
detail. Alternatively, independent or dependent on the amount of local
detail, the factor k3 may be controlled such that it also contributes to
the output image signal SIS. This has the advantage that a low-pass
filtered signal is obtained if much chrominance detail is present and the
original (unfiltered) signal is obtained if a low amount of chrominance
detail is present. Thus, now a selection is possible wherein not only the
low-pass filtered input signal TIS and the mapped input image signal MIS,
but also the input image signal IS itself can contribute to the output
signal.

[0060]FIG. 5 schematically shows a block diagram of an embodiment of the
color mapping system further performing a detail controlled
de-saturation. This block diagram is largely identical to that of FIG. 4.
The only difference is that the de-saturation block 8 has been added to
the branch which provides the input signal IS to the multiplier 7. Thus,
instead of adding a fraction of the input signal IS, now a fraction of
the de-saturated input signal SDI is contributing to the output signal
SIS. The fraction and thus the amount of local de-saturation is
determined by the local detail dependent factor k3. The de-saturation may
be obtained by mixing the luminance intensity of the combined input R, G,
B pixels of the input signal IS with the individual input sub-pixel
intensities R, G, B. The mixing may be a linear addition using weight
factors. The weight factors may be constant or may be controlled by the
local detail detected. Alternatively, the average value of the R, G, B
sub-pixel intensities is mixed with the individual R, G, B, sub-pixel
values. Alternatively, luminance detail (high pass filtered luminance of
the input signal) may be added instead of the luminance itself. The
operation of the system depicted in the block diagram of FIG. 5 is
further elucidated with respect to FIG. 6.

[0061]FIGS. 6A to 6C schematically show an embodiment of mixing factors in
the block diagram of FIG. 5. FIGS. 6A, 6B and 6C show the factors k1, k2
and k3, respectively, as function of the local detail detected. The local
detail is depicted along the horizontal axis and is normalized in the
range zero (no detail) to one (maximum detail which can be displayed). Or
said differently, a low value of the local detail indicates a low content
of high frequencies (or local structure), a high value of the local
detail indicates a high content of high frequencies (or local structure).

[0062]The factor k2 controls the contribution of the mapped input image
signal MIS to the output image signal SIS. This factor k2 is one for
areas with low detail and gradually decreases to zero for areas with
maximum detail. Consequently, the amount of color or gamut mapping
decreases with increasing local detail thereby decreasing artifacts
caused by the color or gamut mapping in areas with high local detail.

[0063]The factor k1 controls the contribution of the low-pass filtered
input signal TIS to the output image signal SIS. If the local detail is
low, the mapper 20 can be fully active without causing artifacts.
Consequently, the factor k1 can be zero for low local detail. If a lot of
local detail is present, the mapper output signal is suppressed and more
low-pass filtered signal TIS is added to the output signal SIS because
the low-passed signal has a sufficiently low resolution to be displayed
without artifacts. Thus, the factor k1 starts increasing from its zero
value at a particular local detail (in the example shown at 0.5) to its
maximum value one at maximum local detail. In an embodiment, the local
detail is local chrominance detail.

[0064]The factor k3 controls the contribution of the saturation decreased
image signal SDI. The factor k3 is zero for low local detail: if no local
detail is present in the input image signal IS, the saturation need not
be decreased. If the local detail increases, the factor k3 increases too
to add more of the saturation decreased image signal SDI to the output
image signal SIS to minimize the artifacts caused by local detail in
saturated backgrounds. At a predetermined value of the local detail, the
contribution of the saturation decreased image signal SDI to the output
signal is decreased with increasing local detail because the chrominance
resolution of the display is too low to display this information and it
is better to use the low-pass filtered image signal TIS. It has to be
noted that optionally, as discussed hereinbefore, also a weighted (the
factor k4) contribution of the input image signal IS can be implemented.

[0065]The amount of de-saturation may be dependent on the detail. For
example, the amount of de-saturation may increase with increasing detail
until a predetermined level of detail. This predetermined detail may be
the maximum chrominance detail which the display is able to display. If
the detail rises above the predetermined level, the de-saturation may
decrease with increasing detail to prevent artifacts in highly detailed
areas.

[0066]FIG. 7 schematically shows a conversion from RGB input samples of
the input image into drive values of sub-pixels of a RGBW display. FIG. 7
explains the conversion, by way of example only, for a particular
configuration of sub-pixels.

[0067]Because the resolution of mobile displays keeps increasing, the
pixel pitch and thus the size of the sub-pixels of the pixel decreases.
However, the electronics in each sub-pixel, such as wiring and thin film
transistor do not scale with the size of the pixels, the aperture of the
sub-pixels decreases even faster than their size. Consequently, the
luminance and thus the power consumption of the backlight must increase
to obtain the same brightness of the image displayed. In conventional
red, green, blue displays (further also referred to as RGB displays),
each sub-pixel comprises a red, green and blue sub-pixel. If a backlight
unit generates white light, for each of the sub-pixel a color filter is
required which maximally is able to transmit only one third of the
impinging white light. The addition of a white sub-pixel to the red,
green and blue sub-pixels may improve the brightness because no color
filter is required for the white (W) sub-pixel and thus the white light
of the backlight unit is substantially completely transmitted. Of course,
with an extra white pixel, only the luminance of unsaturated colors can
be boosted.

[0068]The display pixels have RGBW sub-pixels arranged in a particular
configuration. In the configuration shown in FIG. 7, two input pixels are
displayed on one display pixel: one of the two input pixels is displayed
on the RGB sub-pixels of the display pixel, and the other one of the two
input pixels is displayed on the W sub-pixel. Appropriate sub-pixel
rendering is used in order to provide the same perceived resolution as
conventional RGB striped technology wherein the sub-pixels with the same
color are arranged in columns, and one input pixel is displayed by one
display pixel. This configuration uses only two third of the sub-pixel
columns to obtain, on average, two sub-pixels per pixel and thus provides
a larger pixel aperture than the conventional RGB striped technology.
Note that the present invention has benefits on any RGBW subpixel
configuration, or even on other (RGBX or more general) multi-primary
configurations.

[0069]A conversion system which converts the standard RGB image signal
into drive signals for the RGBW sub-pixels comprises a gamut mapping 2
and a multi-primary conversion 3. The gamut mapping 2 maps the input RGB
gamut GA1 onto the different gamut GA2 which can be represented with the
RGBW sub-pixels. Roughly speaking this mapping boosts the intensity of
unsaturated colors. If the boosted unsaturated color occurs outside the
RGBW gamut GA2, it is clipped to the border (hard clipping) or even
inside (soft clipping) the RGBW gamut GA2. Saturated colors are not
intensity boosted. The multi-primary conversion 3 converts the mapped RGB
values into RGBW drive values suitable for driving the RGBW sub-pixels.
The multi-primary conversion is succeeded by sub-pixel sampling which
halves the number of sub-pixels being driven by the same input pixel. The
sub-pixel sampling method discards the driving value for white (mapping
the RGBW pixel on a RGB sub-pixel triplet), or discards the driving value
for red, green, blue (mapping the RGBW pixel on a white sub-pixel). This
does not affect the luminance resolution, because both the RGB triplet
and the white sub-pixel are used as luminance pixels, but lowers the
chrominance resolution.

[0070]FIG. 7 shows an example of this conversion for a block of four
adjacent RGB input pixels I11, I12, I21, I22 of the input image. Each RGB
input pixel Iij comprises three values Rij, Gij, Bij. The conversion
first performs the mapping 2 and the multi-primary conversion 3 to obtain
the corresponding four RGBW values S11, S12, S21, S22 in the RGBW gamut
GA2. Each one of the four RGBW values Sij comprise four values RIij,
GIij, BIij, and WIij. The set of four RGBW values S11, S12, S21, S22 are
sub-sampled into two RGBW drive signals D12, D22 which each comprise 4
sub-pixel drive values for corresponding sub-pixels RP11, GP11, BP11,
WP11 of a first pixel, and WP21, RP21, GP21, BP21 of a second pixel,
respectively, of the pentile configured display. The sub-sampling selects
the RGB values RI11, GI11, BI11 of the values S11 and the W value W12 of
the values S12 for the first pixel which comprises the sub-pixels RP11,
GP11, BP11, WP11. The sub-sampling selects the RGB values R122, G122,
B122 of the values S22 and the W value W21 of the values S21 for the
first pixel which comprises the sub-pixels WP21, RP21, GP21, BP21.

[0071]The chrominance resolution of such a display is half its luminance
resolution. Both the RGB triplet of sub-pixels and the W sub-pixel
contribute to the luminance, but only the RGB sub-pixels can display
color information. If small text or thin lines (for example one pixel
wide) with saturated colors are present in the input image, detail may
get lost. Or said differently, information in the input image with a
chrominance resolution which is as high as the highest luminance
resolution which can be displayed on the RGBW sub-pixel configuration
cannot be displayed on the RGBW display without artifacts because its
resolution is too high. These artifacts can be minimized by low-pass
filtering the chrominance components (U and V of a YUV signal) of the
input image. Alternatively, the adaptive low-pass filter may be used
which increase the contribution of the low-pass filtered input image
signal if more chrominance detail is detected. This reduces the
chrominance resolution of input images without deteriorating the
luminance resolution. As disclosed in the non-pre-published European
patent application 05110562.5 this low-pass filtering may be controlled
dependent on the local detail in an area comprising the input pixel which
is being processed. However, still artifacts may occur for the special
input signals referred to earlier. In the embodiment discussed with
respect to FIG. 7, these artifacts are decreased by also controlling the
mapping dependent on the local detail.

[0072]FIG. 8 schematically shows a display device comprising the
conversion system. The display device comprises an array 60 of display
pixels which are driven by a select driver 62 and a data driver 64. The
select driver 62 may select the pixels line by line to enable the data
driver 64 to provide the data line-wise to the selected line of pixels.
The RGB input image samples IS which determine the color and intensity of
the input pixels are supplied to a display controller 66. The unit 68
comprises the color mapping unit (the color mapping system in the claims)
which comprises the detail detector 1 and the color mapper 2.
Alternatively, the unit 68 comprises the conversion system which
comprises the color mapping system, the detail detector 1 and the
multi-primary conversion 3. Both the color mapping system and the
conversion system may additionally comprise the local detail controlled
chrominance low-pass filter. The unit 68 may comprise a microprocessor
for implementing the signal processing functions.

[0073]Although in this embodiment, the sub-pixel sampling problem is
described for RGBW displays, it also may exist for other displays,
especially if the resolution of the display is not identical for
luminance and chrominance components. Some examples are RGBx displays
wherein the additional sub-pixel x can have any color, for example yellow
or cyan. The same issue may arise in conventional RGB displays in which
sub-sampling is applied, or in displays wherein a low-pass filtering on
part of the input components of the input pixels is applied.

[0074]Although in this embodiment a particular configuration of the
sub-pixels is shown, the present invention may be relevant to other
implementations in which another configuration of sub-pixels is used.

[0075]It should be noted that the above-mentioned embodiments illustrate
rather than limit the invention, and that those skilled in the art will
be able to design many alternative embodiments without departing from the
scope of the appended claims.

[0077]The invention can be applied to image signals independent on how the
pixel intensity and color are defined. The color data may be converted
into the desired format, for example the RGB format, to be processed in
accordance with the present invention.

[0078]Although the present invention has a wider field of application, the
invention is of particular benefit for displays with lower chrominance
resolution than luminance resolution. This is, for example, true for RGBW
displays, and in particular for displays in which the display is driven
with a sub-sampled set of sub-pixel values. Of course, this approach can
also advantageously used for RGBX displays wherein X is an additional
primary color.

[0079]Local image structure may typically be any spatial relationship
between pixels of related color values, e.g. there may be a texture
present such as e.g. dark grains of a certain size on a lighter local
background. This can be characterized by a measure, e.g. a texture
measure, or some value output from a recognizer (e.g. a class number of
local shape, from a pattern matcher, or a learning system analyzing the
local spatio-color pixel distributions, statistically, semantically,
etc.), etc. This is then converted to a control signal, which may e.g. be
one of a number of values (e.g. high=complex texture; low=simpler
texture), or a continuous curve, or even multidimensional signal (of
course, or a continuous curve, or even multidimensional signal (of course
there may be an additional or comprised mapping so that the final
contrast signal is of the correct magnitude to do the color
transformation, so that e.g. for an average viewer the output picture is
more pleasing).

[0080]In the claims, any reference signs placed between parentheses shall
not be construed as limiting the claim. Use of the verb "comprise" and
its conjugations does not exclude the presence of elements or steps other
than those stated in a claim. The article "a" or "an" preceding an
element does not exclude the presence of a plurality of such elements.
The invention may be implemented by means of hardware comprising several
distinct elements, and by means of a suitably programmed computer. In the
device claim enumerating several means, several of these means may be
embodied by one and the same item of hardware. The mere fact that certain
measures are recited in mutually different dependent claims does not
indicate that a combination of these measures cannot be used to
advantage.